1. Field of the Invention
The present invention relates to an alloy composition for a transmission gear of an automobile containing increased amounts of components for improving the quenching properties, such as nickel, increased amounts of niobium, and reduced amounts of components having a strong oxygen affinity, in order to improve the purity and fatigue strength of the alloy.
2. Description of the Related Art
Chromium, Cr--Mo, and Cr--Mo--Ni steel alloys have been used for transmission gears of automobiles. These alloy compositions attain the desired quenching hardness and hardening depth during the carburizing heat treatment by increasing the content of Cr, Mo, and Ni in a low-steel typically used for manufacturing machine parts. However, these alloys form an abnormal surface layer during the carburizing heat treatment due to the presence of components having a strong oxygen affinity, such as manganese, silicon, and chromium. For this reason, these alloys are limited in their ability to improve the strength of the material to the level required to withstand the increasing stress created by the transmission load during a rise in engine power.
To solve this problem, according to the present invention, the amounts of manganese, silicon, and chromium, which have a strong oxygen affinity and poor quenching properties, were decreased, and the amounts of nickel and molybdenum, which have a weak oxygen affinity, good quenching properties, and desirable structure intensification properties, were increased. Niobium and vanadium were added to restrain the growth of crystal grains.